Abstract

The effects of fish predation on invertebrate community structure and algal biomass were contrasted in a fish-bearing and previously fishless stream in the coast mountains of British Columbia. Predator impact was assessed by stocking coho salmon (Onchorhynchus kisutch) juveniles and prickly sculpin (Cottus asper) in replicate enclosures. Abundance of larger invertebrates (primarily herbivorous mayflies) decreased in the presence of fish, while algal biomass and density of smaller epibenthic herbivores, primarily orthoclad chironomids, correspondingly increased. Effect sizes of fish predation were generally larger in the previously fishless stream than the fish-bearing stream, where invertebrates were presumably better adapted to fish predation. Density of larger invertebrates was lower in the fish-bearing stream, and density of smaller invertebrates and algal biomass were higher, indicating that the effects of fish predation on algal biomass and invertebrate community structure at a larger scale between streams with and without fish are similar to the effects of fish predation at a smaller scale within a single stream.